The present invention relates to fasteners in general, and, more in particular, to fasteners that are permanently installed in a workpiece and which typically cooperate with another fastener to hold objects to the workpiece.
Inserts installed in parent material of a workpiece provide a means for attachment of an object to the workpiece through a second fastener. Inserts are used as when the stress requirements of a connection are greater than that provided by the parent material of the workpiece. An insert can distribute the stress over an area of the workpiece larger than would be available to the second fastener acting alone.
An example of an insert is a plug-like object having male threads for receipt in female threads of the parent material of a workpiece. The insert has an internally threaded bore for receiving male threads of a cooperating fastener. Typically, studs provide an anchor for attachment of objects to parent materials of the workpiece. A stud secures to the workpiece with male threads just like an insert does, but itself provides male threads for cooperation with a female threaded nut or the like.
Obviously, separation of the fastener from parent material of the workpiece is not desired. The threads between the two prevent the fastener from being pulled out of the workpiece. It is not uncommon to also provide against the backing out of the threads of the fastener from the threads of the workpiece.
The provision which prevents the fastener from unthreading from the workpiece has taken several forms. One form has provided radial teeth which plow or broach workpiece material as the fastener is driven essentially axially into a bore in the material. Workpiece material then occupies spaces between the teeth of the fastener and interferes with the teeth to resist relative rotation. This type of arrangement is complicated when the means for attaching the fastener to the workpiece is by threads. Clearly the provision to prevent rotation of the fastener with respect to workpiece cannot come into effect until the fastener is fully threaded into the workpiece. Standoff devices have been used to keep the teeth of the locking device from broaching workpiece material until the fastener is otherwise set. A different technique to accomplish the same result is to drive a locking device into position after the fastener is otherwise set by forcing the locking device axially into a space provided for it. The locking device in the latter technique can be an expander ring to expand material of the fastener radially against the workpiece. Alternatively, the locking device itself can directly contact the workpiece.
Another technique allows the fastener to rotate with respect to a lock ring as the fastener tightens in threads of the workpiece, and the lock ring cannot rotate with respect to the workpiece. When set, the lock ring interferes directly or indirectly with the workpiece to prevent unthreading of the fastener. The interference between the workpiece and the lock ring effected during installation has produced consequential broaching, peening or swedging of the workpiece. These actions on the workpiece can remove protective coatings and subject the workpiece to corrosion. These actions can also create stress risers in the workpiece which ultimately lead to a failure, as through fatigue. Furthermore, the lock employed in this technique may always be under stress, and the structure with which it cooperates may also be under stress in a sense which could lead to failure.